(1) Field of the Invention
The present invention relates to an optical signal receiver apparatus used in an optical signal transmission system. In optical signal transmission systems, usually, a protection (stand-by) system is provided in addition to a working system, and when trouble occurs in the working system, the operation is switched to the protection system. The trouble in such a system is often detected as a loss of an input signal (receiving signal) in an optical receiver apparatus. Since, when trouble occurs in the working system, the above switching from the working system to the protection system is required to be carried out quickly, the loss of an input signal (receiving signal) in an optical receiver apparatus is also required to be detected quickly.
(2) Description of the Related Art
In optical signal receiver apparatuses used in optical signal transmission systems, an optical signal transmitted through an optical transmission line is converted to an electrical signal, and a clock signal is extracted as a receiving clock signal from the electrical signal. Then, data is regenerated from the electrical signal by using the receiving clock signal. In the conventional construction of an optical signal receiver apparatus, a successive-zero detection unit and a level detection unit are provided for detecting the loss of the receiving signal. The level detection unit compares the peak level of the extracted receiving clock signal with a predetermined reference level Vr, and outputs a level alarm signal when the lowering of the peak level under the predetermined reference level Vr is detected by the comparison. The successive-zero detection unit detects a predetermined number of successive zeros in the regenerated data. When the predetermined number of successive zeros is detected in the regenerated data, the successive-zero detection unit outputs an active successive-zero detection signal.
However, in the conventional construction, there is a drawback that it takes hundreds of microseconds to several milliseconds to output the level alarm signal since the above peak level is obtained by an analog peak detection circuit containing a charging circuit, and the successive-zero detection unit can detect the loss of the input only when the receiving clock signal is supplied thereto. Namely, in the conventional optical signal receiver apparatus, generally, the loss of the optical signal input cannot be detected quickly.